Softwood species including radiata pine (“radiata”), southern yellow pine, scots pine, ash, maple, beech, birch, aspen and rubber wood are used in a number of exterior construction applications such as structural members (for example, weight bearers and joists), decking and cladding. Many softwood species, and especially radiata, have disadvantages including lack of dimensional stability in service “stability”, surface checking and poor colour/aesthetics following exposure. These have resulted in softwood timbers being unable to compete in some market applications with more durable and aesthetically pleasing timbers such as Western Red Cedar for cladding and hardwoods such as Kwila (also known as Merbau), for decking. Consequently, some softwood species and radiata pine in particular are viewed as a low-cost commodity material for decking and cladding applications.
When used for decking applications, low cost species such as radiata are often supplied in a form known as “wet, treated” which is very prone to shrinkage as the moisture content of the timber equilibrates to the surrounding conditions. This shrinkage results in unsightly dimensional changes. Furthermore, radiata in particular is not perceived to have pleasing aesthetics in its natural state. However, painting to overcome the lack of aesthetics is not practical for decking applications. Coatings and stains can be applied but these add cost and inconvenience. As a result of these disadvantages, radiata pine in a “wet, treated” format is not considered a premium decking material and is often seen as a cheap decking option by the market.
In cladding applications radiata is generally painted, typically with lighter colours. Painting not only results in a better surface finish, but it also ensures increased stability of the timber and enhances its aesthetic appeal. This is because, if it is not painted, the radiata used for cladding, like the “wet, treated” timber in decking applications, is not always stable and has a propensity to distort (or “warp”) and surface check when exposed to varying climatic conditions.
Use of some softwood species (radiata in particular) in cladding and decking applications does, however, have some advantages, particularly over the hardwoods. These advantages typically relate to cost and to environmental concerns. For example, harder, more durable species (often hardwoods) are often slow growing and exist within naturally-occurring tropical forests resulting in environmental concerns amongst the general public. By comparison, softwood species such as radiata can be grown as plantation species which are quick growing, continually re-forested and, as such, sustainably grown. Radiata or other plantation species are also readily available in most countries. The rapid growth rate of such plantation species compared to non-plantation species such as hardwoods means the trees can be harvested earlier and this, along with ready availability, leads to cost benefits. In addition, prices of many hardwood species are increasing around the world due to their declining availability.
The use of preservatives and timber modification techniques are known to provide means for increasing the durability of soft woods such as radiata. These techniques are well known and provide options individually or together in combination to increase the durability of softwoods such as radiata. 
To improve the stability of timber, it is known to laminate individual boards together. It is known in the art that boards with a vertical grain orientation are more resistant to warping, splitting and surface checking and are thus more stable when used. Laminated wooden boards are typically produced by gluing together individual boards in a flat grain orientation edge to edge or face to face to give individual finished boards with a flat or mixed grain orientation. However, lamination in this way typically results in finished boards of flat or mixed grain orientation that are more prone to surface checking and movement. It also results in unsightly glue lines in the finished product. An example of such lamination techniques is shown in NZ 562263 to Lockwood Buildings Limited.
It is an object of the present invention to provide a method of using timber boards with a substantially flat grain to produce a thermally modified timber product with enhanced dimensional stability and aesthetic characteristics, or at least to provide the public with a useful choice.